1. Field of the Invention
The present invention is related to a gate driving circuit and display module, and more particularly, to a gate driving circuit and display module modulating the scan signal step by step.
2. Description of the Prior Art
A liquid crystal display (LCD) monitor has characteristics of light weight, low power consumption, zero radiation, etc. and is widely used in many information technology (IT) products, such as televisions, mobile phones, and laptop computers. The operating principle of the LCD monitor is based on the fact that different twist states of liquid crystals result in different polarization and refraction effects on light passing through the liquid crystals. Thus, the liquid crystals can be used to control amount of light emitted from the LCD monitor by arranging the liquid crystals in different twist states, so as to produce light outputs at various brightnesses.
Please refer to FIG. 1A, which is a schematic diagram of a thin film transistor (TFT) LCD monitor 10 of the prior art. The LCD monitor 10 includes an LCD panel 100, a source driver 102, a gate driver 104, a voltage generator 106 and a logic control circuit 116. The LCD panel 100 is composed of two substrates, and space between the substrates is filled with liquid crystal materials. One of the substrates is installed with a plurality of data lines 108, a plurality of scan lines (or gate lines) 110 and a plurality of TFTs 112, and another substrate is installed with a common electrode for providing a common signal Vcom outputted by the voltage generator 106. The TFTs 112 are arranged as a matrix on the LCD panel 100. Accordingly, each data line 108 corresponds to a column of the LCD panel 100, each scan line 110 corresponds to a row of the LCD panel 100, and each TFT 112 corresponds to a pixel. Note that the LCD panel 100 composed of the two substrates can be regarded as an equivalent capacitor 114.
The source driver 102 and the gate driver 104 input signals to the corresponding data lines 108 and scan lines 110 based upon a desired image data, to control whether or not to enable the TFT 112 and a voltage difference between two ends of the equivalent capacitor 114, so as to change alignment of the liquid crystals as well as the penetration amount of light. As a result, the desired image data can be correctly displayed on the LCD panel 100. The logic control circuit is utilized for coordinating the source driver 102 and the gate driver 104, such as calibrating timing of source driving signals on the data lines 108 and scan signals on the scan lines 110, such that the TFTs 112 are enabled by the scan signals and receive correct image data via the source driving signals at correct time instances.
Based on manufacturing requirements, components of the driving circuits of the LCD monitor 10 are mainly classified into low voltage devices, medium devices and high voltage devices. The low voltage devices are mainly employed in the logic control circuit 116, and an endurance limit for the low voltage devices is 1.5-1.8V. The medium voltage devices are mainly employed in the source driver 102, and an endurance limit for the medium voltage devices is 5-6 V. The high voltage devices are mainly employed in the gate driver 104, and an endurance limit for the high voltage devices is 25-30 V. Please refer to FIG. 1B, which is schematic diagram of a relationship curve for a conduction current and an operating voltage of a high voltage N-type transistor of the prior. Please also refer to FIG. 1C, which is schematic diagram of a relationship curve for a conduction current and an operating voltage of a high voltage P-type transistor of the prior. When the N-type transistor is enabled, an absolute value of a gate-to-source voltage difference |Vgsn| thereof is equal to 30 V. When the N-type transistor is disabled, the absolute value of the gate-to-source voltage difference |Vgsn| thereof is equal to 0 V. When the P-type transistor is enabled, an absolute value of a gate-to-source voltage difference |Vgsp| thereof is equal to 30 V. When the P-type transistor is disabled, the absolute value of the gate-to-source voltage difference |Vgsp| thereof is equal to 0 V. That is, a full voltage swing of the high voltage devices is 30 V, and the high voltage devices have to endure the full voltage swing without breakdown. Therefore, among the three device categories, the high voltage device require the largest layout area, the most masks and layers in the integrated circuit, and therefore cost the most.
For that reason, the industry focuses on how to employ less high voltage devices in the LCD driving circuits.